The present invention pertains to the field of security systems for documents, including more particularly to novel duplication resistant documents and methods of creating duplication resistant documents.
The importance of making documents safe from duplication and alteration is readily apparent. The advent of improved photocopy equipment, particularly high resolution color photocopy equipment, as well as desk top publishing and digital scanning, has provided the unscrupulous with the means for unauthorized duplication of original documents for the purpose of passing them off, with or without alteration, as the original document. The quality of the reproductions obtainable through these means is so good that, it is difficult to distinguish original copies from color reproductions. Even if the duplication is not exact, the reproduction often appears authentic in the absence of the original for comparison. This problem is well known to the issuers of such original documentation, and considerable attention has been given to find ways and means to prevent unauthorized duplication of such documents by photocopiers or other electronic methods.
Many techniques have been developed to prevent improper reproduction of original documents. One of the more known techniques is based on the phenomenon that photographic copiers have an element value (sometimes referred to as element frequency) threshold above which the photocopier is unable to distinguish the individual elements of the pattern of for example, halftone printing. In general, a pattern with a low line screen value of large sized elements is more easily reproducible than a pattern with a high line screen value of small sized elements.
In accordance with this technique, a hidden warning message, such as xe2x80x9cVOIDxe2x80x9d or xe2x80x9cCOPY,xe2x80x9d is printed in a halftone over a halftone background printed on a substrate. The line screen value of the hidden warning message is selected, such that the halftone elements of the hidden warning message are reproduced when photocopied. The line screen value of the background; however, is selected, such that, the halftone elements of the background are not reproduced when photocopied. As a result, the hidden warning message will appear on duplicates of the original document made by photocopying. This method is also used by reversing the halftones of the hidden warning message and the background such that the elements of the hidden warning message are not reproduced and the elements of the background are reproduced when photocopied or scanned. Known line screen values that are used to print these types of hidden warning messages and backgrounds are, e.g., 65 LPI and 133 LPI, respectively (i.e., a line screen value ratio of approximately 2.)
In addition to selecting differing line screen values for the hidden warning message and the background pattern to allow them to be used to prevent duplication, it is also known to use different respective tonal screen values (i.e., the percentage of ink coverage) can also be selected to differ so that the hidden warning message more easily appears on a reproduction of the original document. Known tonal screen values that are used to print these types of hidden warning messages and background patterns are, e.g., 12% and 10%, respectively (i.e., a tonal screen value ratio of about 1.2).
Because of the disparity between the respective line screen values and respective tonal screen values of the hidden warning message and background pattern, a mere combination of these two techniques would not be effective because the hidden warning message would normally be visible to a casual observer of the original. To minimize the visible appearance of the warning message with this combined technique, the respective tonal screen values are selected so that they are more similar and/or a camouflage pattern can be printed over the hidden warning message and background to obscure the hidden warning message from a casual observer of the original document. The camouflage pattern may be defined by areas in which dots, lines, bars, or marks have been formed for both the hidden warning message and background pattern, or the background pattern may be defined by a pattern of dots, lines, bars, or marks which are smaller than or larger than those used in the hidden warning message and background pattern, or by areas of complete coverage of a paler ink.
A description of these aforementioned techniques can be found in U.S. Pat. Nos. 4,227,720 and 5,197,795.
Another technique and example for creating duplication resistant documents is illustrated in U.S. Pat. Nos. 5,271,645, 5,018,767, and 5,193,853, whereby printed line frequencies are printed at specific angles that mis-register with the protocols of electronic color scanners causing a moire pattern when copied.
While the above techniques have provided some degree of protection of original documents with respect to most copiers, in recent years digital scanners and color copiers have improved substantially. These new color copiers, such as the Canon 700 and 800 series, have made the above techniques less effective in protecting original documents. By manipulating the control settings on such devices, copies can be made of such original documents in which the hidden warning message does not readily appear on reproductions when some of the most commonly used frequency and element size combinations are used. When the contrast setting of these modern photocopiers are set to the lighter settings or the copier is set to a built-in halftone setting, the resolution of the copier is such that it neither reproduces the lower line screen value/high tonal screen value hidden warning message nor the higher line screen value/lower tonal screen value background pattern. If the line screen value and tonal screen value of the hidden warning message is adjusted so that the lower line screen value/high tonal screen value hidden warning message is reproduced at a lighter copier setting, both the higher line screen value/lower tonal screen value background pattern and the lower line screen value/higher tonal screen value hidden warning message are reproduced. In both cases, the hidden warning message does not readily appear on the reproduction of the original document, so that a casual observer of the document may not be alerted that the document they have is not the original.
A greater disparity between the respective line screen values and tonal screen values of the hidden warning message and background pattern would allow the hidden warning message to appear on a reproduction of the original document even with the manipulation of the copier. Due to the great disparity, however, presently known camouflage techniques do not adequately suppress the visual appearance of the hidden warning message on the original document. This could result in the original document being rejected as a copy which is not acceptable to issuers of the original.
There thus remains a need to provide a counterfeit resistant and copy resistant original document and technique that effectively suppresses the visual appearance of a hidden warning message on the original document, while at the same time, effectively causing the hidden message to visibly appear on copies of the original, thereby precluding an unscrupulous copyist from suppressing the hidden warning message on a reproduction of the original document by manipulation of the control settings of the copying or scanning device.
The present invention comprises a novel duplication resistant document and method of producing such a document that when reproduced exhibits a latent message. In a preferred method and embodiment of the present invention, a document comprises a substrate on which a message layer and a camouflaging layer are formed. The message layer comprises a latent message and a background. The contrast between the latent message and the background is such that the latent message is visible on a reproduction of the document. The dynamic camouflaging layer preferably comprises an environmentally varying ink; such as, thermochromic ink, that is formed onto the substrate as a camouflage pattern. The visual density of thermochromic ink inversely varies with temperature; such that, the appearance of the camouflaging layer is different at room temperatures and photocopying or scanning temperatures.
In alternative preferred embodiments, combinations of multi-tone contrasting layers, vignetted contrasting layers, multi-patterned dynamic camouflaging layers, and multi-spectral dynamic camouflaging layers are employed in accordance with the inventive features of the present invention.